1. Field of the Invention
The present invention relates to an imaging lens and an imaging device including the imaging lens. In particular, the present invention relates to an imaging lens and an imaging device including the imaging lens, in which the imaging lens has a two-lens structure that is capable of size and weight reduction and optical performance enhancement. The imaging lens is used in a camera that forms images of objects, such as scenery and human figures, on an image-taking surface of an image sensor element, such as a charge-coupled device (CCD), a complementary metal oxide semiconductor (CMOS), etc. The image sensor element is mounted on a portable computer, a television phone, a portable phone, and the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for cameras that utilize a solid image sensor element, such as the CCD, the CMOS, or the like, that is mounted on a portable phone, a portable computer, a television phone, and the like. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens used in such cameras to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens has been used as such an imaging lens.
However, although a single-lens structure lens system such as this can sufficiently handle application to a solid image sensor element having a resolution of about 110 thousand pixels, called common intermediate format (CIF), the lens system cannot sufficiently utilize the resolution capabilities of a recent solid image sensor element having a high resolution of about 300 thousand pixels, called video graphics array (VGA), and recent solid image sensor elements having high resolutions of one million pixels or more (such as super extended graphics array [SXGA]).
Therefore, conventionally, various two-lens structure lens systems and three-lens structure lens systems that have superior optical performance compared to the single-lens structure lens system are proposed.
The three-lens structure lens system can effectively correct each aberration leading to deterioration of optical performance. Therefore, the three-lens structure lens system can achieve extremely high optical performance. However, the three-lens structure lens system has a large number of components, making size and weight reduction difficult. Production cost also increases because each component requires high precision.
On the other hand, although optical performance that is as high as that of the three-lens structure lens system cannot be expected of the two-lens structure lens system, a higher optical performance than that of the single-lens structure lens system can be achieved. The two-lens structure lens system is compact and suitable for a high-resolution solid image sensor element.
As such a two-lens structure lens system, for example, the lens systems described in the following Patent Literatures 1 and 2 have been used.
[Patent Literature 1] Japanese Patent Unexamined Publication 2001-174701
[Patent Literature 2] Japanese Patent Unexamined Publication 2004-45978
However, the lens system described in Patent Literature 1 is a retro-focus lens system in which a first lens having negative power and a second lens having positive power are set. Therefore, the back focus distance becomes too long. The lens system is not suitable for size and weight reduction.
Furthermore, in the lens system described in Patent Literature 2, a second lens set on an object surface side of a first lens has negative power. Therefore, the lens system is not suitable for size and weight reduction. The second lens has a strong concave surface on an image surface side. Therefore, telecentricity is poor.